Will an e-commerce operator be liable to pay tax in respect of supply of goods or services made through it, instead of actual supplier

Yes, but only in case of services notified under Sec. 9(5) of the CGST Act, 2017. In such cases tax shall be paid by the electronic commerce operator if such services are supplied through it and all the provisions of the Act shall apply to such electronic commerce operator as if he is the supplier
liable to pay tax in relation to the supply of such services. A similar provision for inter-State supply is provided for in Sec. 5(5) of the IGST Act, 2017. (Refer to Notification No. 17/2017- Central Tax (Rate) and 14/2017- Integrated Tax (Rate) dated 28.06.2017)

(FAQ 5: E-COMMERCE)